1. Field of the Invention
The present invention relates to a developer supplier and an image forming apparatus, which employs a supplementary developer, in which a toner and a carrier are pre-mixed in a predetermined proportion, as a developer for a replenishment thereof, and which is compliant with a digital photography apparatus, such as a copier, a fax machine, or a printer, which employs a two-component developer. The present invention also relates to a developer supplying method and an image forming method.
2. Description of the Related Art
Conventionally, with regard to an image forming apparatus that employs a two-component developer, when performing a development operation across an extended period of time, a coating layer of a surface layer of a carrier experiences an abrasion, i.e., is worn away, and therefore white spots may occur within an solid image area. Another circumstance that may arise is that a toner material or an additive may adhere to the coating layer, causing a capability of the toner to become charged to decline gradually as a result, which may cause such as a smudge upon an image forming substrate for printing or imaging, or a toner scattering thereupon. When the developer deteriorates to such an extent as is described herein, it is typically replaced by a periodic visit to an end user thereof by a service personnel. In such a circumstance, the replacement of the developer is performed either upon only the developer, a developer unit including the developer, or an imaging unit including the developer unit.
In a typical market, there is no large variation in types of print source documents that are used by an end user who uses a full color image forming apparatus. In addition, such an end user may be in a circumstance wherein a demand for a printing in full color is appended to a pre-existing demand for a printing in black and white, while taking a consideration of cost and of a speed of such a printout. Consequently, it is more common to print a black and white image, rather than a full color image, with the full color image forming apparatus, and thus, a carrier for a black toner deteriorates more rapidly than a carrier for a color toner. Accordingly, replacements of a black developer are more increased in frequency than replacements of a color developer, which leads to an increase in a down time occurrence as a result of a developer replacement operation, as well as an increase in a running cost of the full color image forming apparatus.
On the other hand, the carrier and the toner are pre-mixed in a toner supplying device, and supplied from the toner supplying device to an developing device, while at the same time, the carrier which has deteriorated in the developing device is removed, to minimize the deterioration of the developer in the developing device and to reduce the frequency of the replacement of the developer (see, as an instance, Japanese Patent Application Publication No. 2004-029306 and Japanese Patent Application Publication No. 2007-133057). The above configuration allows performing the carrier of the developer in the developing device may be replaced while maintaining a volume of the developer in the developing device at a given amount. Such a system including pre-mixing the carrier and the toner in the toner supplying device where the mixture is stored is referred to as a “pre-mixed toner system”.
Employing the pre-mixed toner system reduces a speed at which the carrier deteriorates, thus extending a period of an interval between the developer replacements. As a consequence, however, a configuration of the image forming apparatus is made more complex, while on the other hand, the image forming apparatus need not be significantly increased in a size thereof, the frequency of the replacement of the developer is reduced, and it is possible to achieve a shortening of a time for an operation of the replacement of the developer, as well as to control an increase in the running cost of the full color image forming apparatus.
In a circumstance where the pre-mixed toner system is adopted, the system is naturally adopted for the developer for every color (see, as an instance, Japanese Patent Application Publication No. 2004-029306). In such an image forming apparatus, it is preferable to perform a replacement of the developer for every color at one time, in order to control the increase in the down time that arises from the operation of the replacement of the developer, as well as the increase in the running cost. When adopting the conventional pre-mixed toner system, however, the interval for the replacement of the developer varies for each color, and thus, it has not been preferable to perform the operation of the replacement of the developer all at one time because, if every color is thus replaced all at one time, the developer not reaching its end of life is replaced as well as the developer that reaching its end of life.
Thus, a technology in which volume of the black developer is made larger than volume of the color developer has been developed and commercialized. Simply increasing the volume of the developer, however, causes such a problem as being unable to use a common toner cartridge to hold both the black toner and the color toner, as well as a problem of causing an increase in a size of the toner cartridge for the black toner larger, as well as potentially causing a further increase in a size of the image forming apparatus itself, as a result.
As a technology that is capable of minimizing the increase in the size of the image forming apparatus, there is known a technology in which a thickness of a film of the black developer on a development roller is made greater than a thickness of a film of the color developer. In such a circumstance, however, since a physical property of the carrier is changed in order to achieve a standardization of the developing device, an impact upon an image quality may potentially occur, i.e., the image quality may decline as a result.